Fluoropolymer dispersions containing no or little low molecular weight fluorinated surfactant

ABSTRACT

The present invention provides a fluoropolymer dispersion comprising fluoropolymer particles having an average particle size of 10 to 400 nm dispersed in water whereby the dispersion is free of fluorinated surfactant having a molecular weight of less than 1000 g/mol or contains the fluorinated surfactant having a molecular weight of less than 1000 g/mol in an amount of not more than 0.025% by weight based on the total weight of solids in the dispersion. The dispersion further comprises a non-ionic surfactant and an anionic surfactant selected from fluorinated anionic surfactants having a molecular weight of at least 1000 g/mol, non-fluorinated anionic surfactants and mixtures thereof.

1. FIELD OF THE INVENTION

[0001] The present invention relates to aqueous fluoropolymerdispersions that are free of low molecular weight fluorinated surfactantor that contain the latter in low amounts. In particular, the presentinvention relates to reducing the viscosity of such fluoropolymerdispersions that are high in solids content and that contain non-ionicsurfactants as a stabilizer.

2. BACKGROUND OF THE INVENTION

[0002] Fluoropolymers, i.e. polymers having a fluorinated backbone, havebeen long known and have been used in a variety of applications becauseof several desirable properties such as heat resistance, chemicalresistance, weatherability, UV-stability etc . . . . The variousfluoropolymers are for example described in “Modern Fluoropolymers”,edited by John Scheirs, Wiley Science 1997. The fluoropolymers may havea partially fluorinated backbone, generally at least 40% by weightfluorinated, or a fully fluorinated backbone. Particular examples offluoropolymers include polytetrafluoroethylene (PTFE), copolymers oftetrafluoroethylene (TFE) and hexafluoropropylene (HFP) (FEP polymers),perfluoroalkoxy copolymers (PFA), ethylene-tetrafluoroethylene (ETFE)copolymers, terpolymers of tetrafluoroethylene, hexafluoropropylene andvinylidene fluoride (THV) and polyvinylidene fluoride polymers (PVDF).

[0003] The fluoropolymers may be used to coat substrates to providedesirable properties thereto such as for example chemical resistance,weatherability, water- and oil repellency etc . . . . For exampleaqueous dispersions of fluoropolymer may be used to coat kitchen ware,to impregnate fabric or textile e.g. glass fabric, to coat paper orpolymeric substrates. For sake of economy and convenience, thefluoropolymer dispersions will typically have between 30% by weight and70% by weight of fluoropolymer solids.

[0004] A frequently used method for producing aqueous dispersions offluoropolymers involves aqueous emulsion polymerization of one or morefluorinated monomers usually followed by an upconcentration step toincrease the solids content of the raw dispersion obtained after theemulsion polymerization. The aqueous emulsion polymerization offluorinated monomers generally involves the use of a fluorinatedsurfactant. Frequently used fluorinated surfactants includeperfluorooctanoic acids and salts thereof, in particular ammoniumperfluorooctanoic acid. Further fluorinated surfactants used includeperfluoropolyether surfactants such as disclosed in EP 1059342, EP712882, EP 752432, EP 816397, U.S. Pat. No. 6,025,307, U.S. Pat. No.6,103,843 and U.S. Pat. No. 6,126,849. Still further surfactants thathave been used are disclosed in U.S. Pat. No. 5,229,480, U.S. Pat. No.5,763,552, U.S. Pat. No. 5,688,884, U.S. Pat. No. 5,700,859, U.S. Pat.No. 5,804,650, U.S. Pat. No. 5,895,799, WO 00/22002 and WO 00/71590.

[0005] Most of these fluorinated surfactants have a low molecularweight, i.e. a molecular weight of less than 1000 g/mol. Recently, suchlow molecular weight fluorinated compounds have raised environmentalconcerns. Accordingly, measures have been taken to either completelyeliminate the fluorinated low molecular weight surfactants from aqueousdispersion or at least to minimize the amount thereof in an aqueousdispersion. For example, WO 96/24622 and WO 97/17381 disclose an aqueousemulsion polymerization to produce fluoropolymers whereby thepolymerization is carried out without the addition of fluorinatedsurfactant. U.S. Pat. No. 4,369,266 on the other hand discloses a methodwhereby part of fluorinated surfactant is removed throughultrafiltration. In the latter case, the amount of fluoropolymer solidsin the dispersion is increased as well, i.e. the dispersion isupconcentrated while removing fluorinated surfactant. WO 00/35971further discloses a method in which the amount of fluorinated surfactantis reduced by contacting the fluoropolymer dispersion with an anionexchanger.

[0006] Since the solids content of the raw dispersions immediately afteremulsion polymerization is usually in the range of upto 35% by weight,the raw dispersions are subjected to an upconcentration process so as toincrease the solids content thereof. In order to preserve the stabilityof the dispersion, the upconcentration typically is carried out in thepresence of a stabilizer, in particular a non-ionic surfactant that actsas a stabilizer.

[0007] However, when fluoropolymer dispersions that contain no or only asmall amount of fluorinated low molecular weight surfactant areupconcentrated, it was found that a viscosity increase results which maybe unacceptable. Moreover, the stability of the upconcentrateddispersions may under certain conditions be inferior to dispersions inwhich the amount of low molecular weight fluorinated surfactant ishigher.

[0008] Accordingly, there exists a desire to remove one or more of theaforementioned disadvantages of the prior art.

3. SUMMARY OF THE INVENTION

[0009] According to one aspect of the present invention, there isprovided a fluoropolymer dispersion comprising fluoropolymer particleshaving an average size of 10 nm to 400 nm dispersed in water whereby thedispersion is free of fluorinated surfactant having a molecular weightof less than 1000 g/mol or contains the fluorinated surfactant having amolecular weight of less than 1000 g/mol in an amount of not more than0.025% by weight based on the total weight of solids in the dispersion.The dispersion further comprises a non-ionic surfactant and an anionicsurfactant selected from fluorinated anionic surfactants having amolecular weight of at least 1000 g/mol, non-fluorinated anionicsurfactants and mixtures thereof.

[0010] According to a further aspect, the invention also provides amethod of providing a fluoropolymer particle dispersion comprising thesteps of:

[0011] providing a fluoropolymer dispersion comprising fluoropolymerparticles having an average size of 10 nm to 400 nm comprisingfluorinated surfactant having a molecular weight of less than 1000 g/molor being free thereof;

[0012] reducing the amount of the fluorinated surfactant in thedispersion if the amount thereof is more than 0.025% by weight based onthe total weight of solids of the dispersion, preferably based on thetotal weight of fluoropolymer solids in the dispersion;

[0013] upconcentrating the fluoropolymer dispersion in the presence of anon-ionic surfactant so as to increase the amount of fluoropolymersolids in the dispersion; and

[0014] adding an anionic surfactant selected from fluorinated anionicsurfactants having a molecular weight of at least 1000 g/mol,non-fluorinated anionic surfactants and mixtures thereof, to thefluoropolymer dispersion prior to or after upconcentrating thefluoropolymer dispersion.

[0015] Still further, the present invention provides a method of coatingsubstrates with the aforementioned fluoropolymer dispersion of theinvention.

4. DETAILED DESCRIPTION OF THE INVENTION

[0016] In accordance with the present invention it was found that aviscosity increase of fluoropolymer dispersions that contain a non-ionicsurfactant and that are free of low molecular weight fluorinatedsurfactant or that contain the latter in low amounts, e.g. less than0.025% by weight (based on the total weight of solids, in particular onthe total weight of fluoropolymer solids in the dispersion), preferablynot more than 0.01% by weight and most preferably less than 0.005% byweight, can be reduced or avoided if the fluoropolymer dispersioncontains an anionic non-fluorinated surfactant, an anionic fluorinatedsurfactant having a molecular weight of at least 1000 g/mol (hereinaftercalled high molecular weight fluorinated surfactant) or a mixturethereof. Furthermore, the stability of the fluoropolymer dispersion mayalso be improved by the addition of the anionic non-fluorinatedsurfactant or anionic high molecular weight fluorinated surfactant.

[0017] Preferred anionic non-fluorinated surfactants are surfactantsthat have an acid group that has a pK_(a) of not more than 4, preferablynot more than 3. It was found that such anionic surfactants in additionto controlling the viscosity, are generally also capable of increasingthe stability of the fluoropolymer dispersion. Examples ofnon-fluorinated anionic surfactants include surfactants that have one ormore anionic groups. Anionic non-fluorinated surfactants may include inaddition to one or more anionic groups also other hydrophilic groupssuch as polyoxyalkylene groups having 2 to 4 carbons in the oxyalkylenegroup, such as polyoxyethylene groups, or groups such as such as anamino groups. Nevertheless, when amino groups are contained in thesurfactant, the pH of the dispersion should be such that the aminogroups are not in their protonated form. Typical non-fluorinatedsurfactants include anionic hydrocarbon surfactants. The term “anionichydrocarbon surfactants” as used herein comprises surfactants thatcomprise one or more hydrocarbon moieties in the molecule and one ormore anionic groups, in particular acid groups such as sulphonic,sulfuric, phosphoric and carboxylic acid groups and salts thereof.Examples of hydrocarbon moieties of the anionic hydrocarbon surfactantsinclude saturated and unsaturated aliphatic groups having for example 6to 40 carbon atoms, preferably 8 to 20 carbon atoms. Such aliphaticgroups may be linear or branched and may contain cyclic structures. Thehydrocarbon moiety may also be aromatic or contain aromatic groups.Additionally, the hydrocarbon moiety may contain one or more heteroatoms such as for example oxygen, nitrogen and sulfur.

[0018] Particular examples of anionic hydrocarbon surfactants for use inthis invention include alkyl sulfonates such as lauryl sulfonate, alkylsulfates such as lauryl sulfate, alkylarylsulfonates andalkylarylsulfates, fatty (carboxylic) acids and salts thereof such aslauric acids and salts thereof and phosphoric acid alkyl or alkylarylesters and salts thereof. Commercially available anionic S hydrocarbonsurfactants that can be used include Emulsogen™ LS (sodium laurylsulfate) and Emulsogen™ EPA 1954 (mixture of C₁₂ to C₁₄ sodium alkylsulfates) available from Clariant GmbH and TRITON™ X-200 (sodiumalkylsulfonate) available from Union Carbide. Preferred are anionichydrocarbon surfactants having a sulfonate group.

[0019] Other suitable anionic non-fluorinated surfactants includesilicone based surfactants such as polydialkylsiloxanes having pendinganionic groups such as phosphoric acid, groups, carboxylic acid groups,sulfonic acid groups and sulfuric acid groups and salts thereof.

[0020] Alternative to or in addition to the anionic non-fluorinatedsurfactant, a high molecular weight fluorinated surfactant can be used.The high molecular weight fluorinated surfactant has a molecular weightof at least 1000 g/mol, preferably at least 1200 g/mol. Examples of highmolecular weight anionic and fluorinated surfactants comprise polymericsurfactants and include perfluoropolyethers having one or more anionicgroups such as carboxylic acid groups or salts thereof Examples ofperfluoropolyether surfactants include those according to the followingformulas (I) or (II):

R_(f) ^(a)—O—(CF₂O)_(k)(CF₂CF₂O)_(p)(CF(CF₃)CF₂O)_(q)-Q¹-COOM   (I)

MOOC-Q¹-O—(CF₂O)_(k)(CF₂CF₂O)_(p)(CF(CF₃)CF₂O)_(q)-Q²-COOZ   (II)

[0021] wherein k, p and q each represent a value of 0 to 15, typically 0to 10 or 12 and the sum of k, p and q being such that the number averagemolecular weight is at least 1000 g/mol, R_(f) ^(a) represents aperfluoroalkyl group preferably of 2 to 4 carbon atoms, M and Z eachindependently represent hydrogen or a cation, preferably a monovalentcation such as ammonium or an alkali metal ion and Q¹ and Q² eachindependently represents —CF₂— or —CF(CF₃)—.

[0022] Examples of fluorinated surfactants of formula (II) include thosecorresponding to the formula:

R_(f) ^(a)—O—(CFXCF₂O)_(r)—CFX—COOM   (III)

[0023] wherein R_(f) ^(a) and M have the meaning as defined in formula(II), X is a hydrogen atom or a fluorine atom and r has a value suchthat the molecular weight of the surfactant is at least 1000 g/mol.Examples of such fluorinated surfactants are disclosed in EP 219065.

[0024] Still further fluorinated polymeric surfactants that can be usedinclude the perfluoropolymers that comprise repeating units of theformula:

[0025] wherein s is 0, 1 or 2, and t is an integer of 2 to 4, and G is amoiety containing one or more anionic groups. Examples of suitableanionic groups include: carboxyl groups, e.g., —CO₂M where M may behydrogen, a mono or divalent metal ion (e.g., sodium, potassium ormagnesium), ammonium (e.g., simple ammonium, tetraalkylammonium,tetraarylammonium) or phosphonium (e.g., tetraalkylphosphonium); orsulfonate groups, e.g., —SO₃M, where M is defined as above. Preferably,the fluorinated polymeric surfactant is a copolymer having units derivedfrom tetrafluoroethylene and units according to formula (IV). Suchcopolymers and their method of making are disclosed in for example U.S.Pat. No. 5,608,022 and WO 00/52060. Suitable fluorinated polymericsurfactants are available as Nafion™ superacid catalysts from E. I.duPont de Nemours & Co., Wilmington, Del. and are also available asFlemion™ superacid polymers from Asahi Chemical Co., Osaka, Japan and asAcipex™ superacid polymers from Asahi Glass Co., Tokyo, Japan.

[0026] The amount of anionic surfactant added to the fluoropolymerdispersion will generally depend on the nature of the fluorinatedsurfactant, nature and amount of the fluoropolymer, nature and amount ofnon-ionic surfactant present in the dispersion and nature and amount oflow molecular weight fluorinated surfactant that may be present in thefluoropolymer dispersion. Typically, the amount of anionic surfactantwill be between 10 ppm and 5000 ppm, preferably between 100 ppm and 3000ppm, more preferably between 100 ppm and 2500 ppm based on the weight ofthe fluoropolymer solids in the dispersion. When too low amounts of theanionic surfactant are used, an undesirable viscosity increase may stillbe observed. On the other hand, when too large amounts of the anionicsurfactant are added the viscosity may also raise. If it is furtherdesired or needed to increase the stability of the dispersion, it may benecessary to use the anionic surfactant in an amount of at least 2000ppm based on the weight of fluoropolymer solids. The optimalconcentration of the anionic surfactant in the dispersion can be easilydetermined by one skilled in the art through routine experimentation.

[0027] The low molecular weight fluorinated surfactant, when present, may be any of the low molecular weight fluorinated surfactants that canbe used in the emulsion polymerization of fluorinated monomers andinclude in particular those that have been mentioned above in respect ofthe discussion of the prior art. Commonly used low molecular weightfluorinated surfactants are telogenic and include those that correspondto the formula:

Y—R_(f)-Z-M   (V)

[0028] wherein Y represents hydrogen, Cl or F; R_(f) represents a linearor branched perfluorinated alkylene having 4 to 10 carbon atoms; Zrepresents COO⁻or SO₃ ⁻ and M represents a monovalent cation such as analkali metal ion or an ammonium ion.

[0029] The low molecular weight fluorinated surfactant, when present inthe fluoropolymer dispersion, may be present in amounts of less than0.025% by weight, preferably not more than 0.01% by weight and mostpreferably not more than 50 ppm based on the total amount of solids inthe dispersion.

[0030] The fluoropolymer dispersion of the present invention alsoincludes a non-ionic surfactant. The non-ionic surfactant is generally anon-fluorinated non-ionic surfactant. Typically, the non-ionicsurfactant is a surfactant that contains one or more hydrocarbonmoieties, e.g. as described above, linked to a non-ionic hydrophilicgroup. The non-ionic hydrophilic group generally comprises oxyalkylenegroups in which the alkylene group has 2, 3 or 4 carbon atoms. Forexample, the non-ionic hydrophilic group may be a polyoxyethylene group,a polyoxypropylene group or a copolymer, including block-copolymers,comprising oxyethylene and oxypropylene groups. According to aparticular embodiment in connection with the present invention, thenon-ionic surfactant corresponds to the formula:

R¹—O—[CH₂CH₂O]_(n)—[R²O]_(m)—R³   (VI)

[0031] wherein R¹ represents an aromatic or aliphatic hydrocarbon grouphaving at least 8 carbon atoms, R² represents an alkylene having 3carbon atoms, R³ represents hydrogen or a C₁-C₃ alkyl group, n has avalue of 0 to 40, m has a value of 0 to 40 and the sum of n+m being atleast 2.

[0032] It will be understood that in the above formula (VI), the unitsindexed by n and m may appear as blocks or they may be present in analternating or random configuration.

[0033] Examples of non-ionic surfactants according to formula (VI) aboveinclude alkylphenol oxy ethylates of the formula:

[0034] wherein R is an alkyl group of 4 to 20 carbon atoms and rrepresents a value of 4 to 20. Examples of surfactants according toformula (VII) include ethoxylated p-isooctylphenol commerciallyavailable under the brand name TRITON™ such as for example TRITON™ X 100wherein the number of ethoxy units is about 10 or TRITON™ X 114 whereinthe number of ethoxy units is about 7 to 8.

[0035] Still further examples include those in which R¹ in the aboveformula (VI) represents an alkyl group of 4 to 20 carbon atoms, m is 0and R³ is hydrogen. An example thereof includes isotridecanolethoxylated with about 8 ethoxy groups and which is commerciallyavailable as GENAPOL® X 080 from Clariant GmbH. Non-ionic surfactantsaccording to formula (VI) in which the hydrophilic part comprises ablock-copolymer of ethoxy groups and propoxy groups may be used andwell. Such non-ionic surfactants are commercially available fromClariant GmbH under the trade designation GENAPOL® PF 40 and GENAPOL® PF80.

[0036] The non-ionic surfactant is generally present in thefluoropolymer dispersion in an amount of 1% by weight to 12% by weightrelative to the total weight of solids in the fluoropolymer dispersion.Preferably the amount is between 3% by weight and 10% by weight.

[0037] The fluoropolymer contained in the fluoropolymer dispersion is apolymer that has a partially or fully fluorinated backbone. Typicallythe fluoropolymer is a polymer that has a backbone that is at least 40%by weight fluorinated, preferably at least 50% by weight, morepreferably at least 60% by weight. The fluoropolymer may also have afully fluorinated backbone such as for example in PTFE. Thefluoropolymer may be a homo- or copolymer and the dispersion may containa mixture of different fluoropolymers. Examples of fluoropolymersinclude copolymers of tetrafluoroethylene which can be processed fromthe melt, especially those of tetrafluoroethylene/hexafluoropropylene,tetrafluoroethylene/perfluoro(alkylvinyl) ethers with perfluoroalkylradicals having 1 to 5 C atoms, in particular perfluoro(n-propyl-vinyl)ethers, tetrafluoroethylene/ethylene,tetrafluoroethylene/trifluorochloroethylene,trifluorochloroethylene/ethylene, tetrafluoroethylene/vinylidenefluoride and hexafluoropropylene/vinylidene fluoride, and terpolymers oftetrafluoroethylene/perfluoro(alkylvinyl) ether/hexafluoropropylene,tetrafluoroethylene/ethylene/hexafluoropropylene andtetrafluoroethylene/vinylidene fluoride/hexafluoropropylene, or ofquaternary polymers of tetrafluoroethylene/vinylidenefluoride/hexafluoropropylene/perfluoro(alkylvinyl) ethers andtetrafluoroethylene/ethylene/hexafluoropropylene/perfluoro(alkylvinyl)ethers. Further fluoropolymers that can be used in the dispersioninclude polyvinyl fluoride, polyvinylidene fluoride andpolytrifluorochloroethylene. The dispersion may also comprise polymersthat cannot be processed from the melt such as polytetrafluoroethylene,that is to say of the homopolymer and which can optionally containmodifying comonomers, such as hexafluoropropylene orperfluoro(alkylvinyl) ethers or chlorotrifluoroethylene, in smallproportions (0.1 to 3 mol %).

[0038] The average particle size (average particle diameter) of thefluoropolymer in the dispersion is generally in the range of 10 nm to400 nm, preferably between 25 nm and 400 nm. The average particlediameter is generally determined through dynamic light scattering and anumber average particle diameter may thereby be determined. Thedispersion may be mono-modal as well as multi-modal such as bimodal. Theamount of fluoropolymer in the dispersion is typically at least 30% byweight, for example between 35% by weight and 70% by weight.

[0039] The fluoropolymer dispersions can be used to coat a substrate.For example the fluoropolymer dispersions may be used to coat a metalsubstrate, polymeric substrates such as polyester and polypropylenesubstrates or to coat paper. The fluoropolymer dispersions may also beused to coat or impregnate textile or fabrics, in particular glass fibersubstrates. Before coating, the fluoropolymer dispersion may be mixedwith further ingredients to prepare a coating composition as may bedesired for the particular coating application. For example, thefluoropolymer dispersion may be combined with polyamide imide andpolyphenylene sulfone resins as disclosed in for example WO 94/14904 toprovide anti-stick coatings on a substrate. Further coating ingredientsinclude inorganic fillers such as colloidal silica, aluminium oxide, andinorganic pigments as disclosed in for example EP 22257 and U.S. Pat.No. 3,489,595.

[0040] The fluoropolymer dispersions are generally obtained by startingfrom a so-called raw dispersion, which may result from an emulsionpolymerization of fluorinated monomer. Such dispersion may be free oflow molecular weight fluorinated surfactant if the polymerization hasbeen conducted in the absence of a low molecular weight fluorinatedsurfactant but will generally contain substantial amounts of lowmolecular weight fluorinated surfactant. If the concentration of lowmolecular weight fluorinated surfactant in the dispersion is more than adesired level, e.g. above 0.025% by weight, at least part thereof shouldbe removed.

[0041] According to one embodiment to reduce the amount of low molecularweight of fluorinated surfactant, a non-ionic surfactant, e.g. asdisclosed above is added to the fluoropolymer dispersion and thefluoropolymer dispersion is then contacted with an anion exchanger. Sucha method is disclosed in detail in WO 00/35971. Suitable anionexchangers include those that have a counterion corresponding to an acidhaving a pK_(a) value of at least 3.

[0042] The anion exchange process is preferably carried out inessentially basic conditions. Accordingly, the ion exchange resin willpreferably be in the OH⁻ form although anions like fluoride or oxalatecorresponding to weak acids may be used as well. The specific basicityof the ion exchange resin is not very critical. Strongly basic resinsare preferred because of their higher efficiency in removing the lowmolecular weight fluorinated surfactant. The process may be carried outby feeding the fluoropolymer dispersion through a column that containsthe ion exchange resin or alternatively, the fluoropolymer dispersionmay be stirred with the ion exchange resin and the fluoropolymerdispersion may thereafter be isolated by filtration. With this method,the amount of low molecular weight fluorinated surfactant can be reducedto levels below 150 ppm or even below 10 ppm. Accordingly, dispersionssubstantially free of low molecular weight fluorinated surfactant maythereby be obtained.

[0043] In case the low molecular weight fluorinated surfactant is in itsfree acid form is steam-volatile, the following method may be used toreduce the amount of low molecular weight fluorinated surfactant. Asteam-volatile fluorinated surfactant in its free acid form may beremoved from aqueous fluoropolymer dispersions, by adding a nonionicsurfactant to the aqueous fluoropolymer dispersion and, at a pH-value ofthe aqueous fluoropolymer dispersion below 5, removing thesteam-volatile fluorinated surfactant by distillation until theconcentration of steam-volatile fluorinated surfactant in the dispersionreaches the desired value. Low molecular weight fluorinated surfactantthat can be removed with this process include for example thesurfactants according to formula (V) above.

[0044] It will generally be desirable to increase the amount offluoropolymer solids in the dispersion. To increase the amount offluoropolymer solids, any of the upconcentration techniques may be used.These upconcentration techniques are typically carried out in thepresence of a non-ionic surfactant which is added to stabilize thedispersion in the upconcentration process. The amount of non-ionicsurfactant that should generally be present in the dispersion forupconcentration is typically between 1% by weight and 12% by weight,preferably between 3% by weight and 10% by weight. Suitable methods forupconcentration include ultrafiltration, thermal upconcentration,thermal decantation and electrodecantation as disclosed in GB 642,025.

[0045] The method of ultrafiltration comprises the steps of (a) addingnon-ionic surfactant to a dispersion that desirably is to beupconcentrated and (b) circulating the dispersion over a semi-permeableultra-filtration membrane to separate the dispersion into a fluorinatedpolymer dispersion concentrate and an aqueous permeate. The circulationis typically at a conveying rate of 2 to 7 meters per second andeffected by pumps which keep the fluorinated polymer free from contactwith components which cause frictional forces. The method ofultrafiltration further has the advantage that during upconcentrationalso some low molecular weight fluorinated surfactant is removed.Accordingly, the method of ultrafiltration may be used to simultaneouslyreduce the level of low molecular weight fluorinated surfactant andupconcentrate the dispersion.

[0046] To increase the fluoropolymer solids in the aqueous dispersion,thermal decantation may also be employed. In this method, a non-ionicsurfactant is added to the fluoropolymer dispersion that is desirablyupconcentrated and the dispersion is then heated so as to form asupernatant layer that can be decanted and that typically contains waterand some non-ionic surfactant while the other layer will contain theconcentrated dispersion. This method is for example disclosed in U.S.Pat. No. 3,037,953 and EP 818506.

[0047] Thermal upconcentration involves heating of the dispersion andremoval of water under a reduced pressure until the desiredconcentration is obtained.

[0048] In accordance with the present invention, the anionic surfactantto control viscosity is added prior to or after the upconcentrationdepending on the method of upconcentration used. For example, ifultrafiltration is used, it will generally be preferred to add theanionic surfactant subsequent to the upconcentration to avoid lossthereof in the ultrafiltration. If the thermal upconcentration method isused, the anionic surfactant can be added prior to the upconcentrationas well as subsequent to the upconcentration.

EXAMPLES

[0049] Abbreviations:

[0050] PTFE=polytetrafluoroethylene

[0051] APFOA=ammonium salt of perfluorooctanoic acid

[0052] TRITON™ X-100=ethoxylated p-isooctylphenol non-ionic surfactant

[0053] EMULSOGEN™ LS=sodium lauryl sulfate

[0054] TRITON™ X-200=sodium alkylarylpolyether sulfonate

[0055] Test methods:

[0056] The viscosity of the dispersions was measured using a BrookfieldRheometer DV-III, spindel 86 at 20° C. and 20 D/1/s.

[0057] Stability Test:

[0058] The fluoropolymer dispersion was mixed and agitated withadditional components to formulate a coating composition as disclosed inEP 894541. To this end, the fluoropolymer dispersion was mixed with anaqueous composition containing a polyamideimide resin (PAI) such thatthe weight ratio of fluoropolymer solids to PAI solids was 1:1.

[0059] Mixing was carried out with a blade agitator at 800 rpm. The timeuntil coagulation occurred was noted.

Comparative Example 1

[0060] A fluoropolymer dispersion of PTFE of a particle size of about220 nm and having a solids content between 23% by weight was obtainedfrom an emulsion polymerization. To the dispersion were added 6% byweight of TRITON™ X-100. The dispersion contained about 0.1% by weightof APFOA based on total weight of the dispersion (=4350 ppm based onpolymer solids). The dispersion was upconcentrated throughultrafiltration to an amount of PTFE solids of 60% by weight. Theresulting dispersion had a viscosity of 20 mPa.

Comparative Example 2

[0061] The procedure of comparative example 1 was repeated except thatthe dispersion obtained after the emulsion polymerization was contactedwith an anion exchange resin so as to reduce the amount of APFOA in thedispersion to 7 ppm based on total weight of the dispersion (=30ppmbased on polymer solids). This dispersion was then upconcentrated asdescribed in comparative example 1. It was found that the viscosity ofthe dispersion was increased to 101 mPa. The dispersion had a too highviscosity for coating substrates such as metal substrates or glass clothbecause of bubble building.

Example 1

[0062] To the dispersion obtained in comparative example 2 afterupconcentration, there was added 2000 ppm based on the solids amount ofEMULSOGEN™ LS. The viscosity of the dispersion thereby decreased to 16.7mPa. The dispersion thus obtained is suitable for coating for examplemetal substrates.

Example 2

[0063] To the dispersion obtained in comparative example 2 afterupconcentration, there was added 1500 ppm based on the solids amount ofTRITON™ X-200. The viscosity of the dispersion thereby decreased to 18mPa. The dispersion thus obtained is suitable for coating for examplemetal substrates.

Comparative Example 3

[0064] A PTFE dispersion having 7 ppm APFOA was upconcentrated to 58%solids in the presence of 5% of TRITON™ X-100. The obtained dispersionwas tested for stability. Immediately coagulation occurred.

Example 3

[0065] A dispersion was produced as in example I but with the differencethat only 1500 ppm of EMULSOGEN™ LS was used. Coagulation occurred afterabout 1 hour.

Example 4

[0066] A dispersion was produced as in example 1 but with the differencethat only 3000 ppm of EMULSOGEN™ LS was used. No coagulation occurredduring at least 20 hours of agitation.

Comparative Example 4

[0067] A dispersion of PTFE containing 7 ppm of APFOA (based on totalweight of the dispersion) and 8.5% by weight based on total weight ofsolids, of non-ionic surfactant was prepared. The dispersion had asolids amount of 59% by weight based on the total weight of thedispersion. The viscosity of this dispersion was 275 mPas and as aresult, coating of glass cloth was not possible because of airentrapment.

Example 5

[0068] A PTFE dispersion as in comparative example 4 was prepared and3000 ppm (based on total solids) of EMULSOGEN™ LS were added. Theviscosity of this dispersion was only 37 mPas allowing the coating ofglass cloth without air entrapment.

1. Fluoropolymer dispersion comprising fluoropolymer particles having an average particle size of 10 to 400 nm dispersed in water, said dispersion being free of fluorinated surfactant having a molecular weight of less than 1000 g/mol or containing said fluorinated surfactant having a molecular weight of less than 1000 g/mol in an amount of not more than 0.025% by weight based on the total weight solids of said dispersion, said dispersion further comprising a non-ionic surfactant characterized in that said dispersion contains an anionic surfactant selected from fluorinated anionic surfactants having a molecular weight of at least 1000 g/mol, non-fluorinated anionic surfactants and mixtures thereof.
 2. Fluoropolymer dispersion according to claim 1 wherein the anionic surfactant is present in an amount of 100 to 5000 ppm based on the total weight of solids in the dispersion.
 3. Fluoropolymer dispersion according to claim 1 or 2 wherein the anionic surfactant comprises a non-fluorinated anionic surfactant comprising an acid group having a pK_(a) of less than
 4. 4. Fluoropolymer dispersion according to any of the previous claims, wherein said dispersion comprises said fluoropolymer particles in an amount of at least 30% by weight based on the total weight of the dispersion.
 5. Fluoropolymer dispersion according to claim 4 wherein the amount of fluoropolymer particles is between 35% by weight and 70% by weight.
 6. Fluoropolymer dispersion according to any of the previous claims wherein the non-ionic emulsifier corresponds to the formula: R¹—O—[CH₂CH₂O]_(n)—[R²O]_(m)—R³ wherein R¹ represents an aromatic or aliphatic hydrocarbon group having at least 8 carbon atoms, R² represents an alkylene having 3 carbon atoms, R³ represents hydrogen or a C₁-C₃ alkyl group, n has a value of 0 to 40, m has a value of 0 to 40 and the sum of n+m being at least 2:
 7. Fluoropolymer dispersion according to any of the previous claims wherein the amount of non-ionic emulsifier is between 1 % and 12% by weight relative to the total weight of solids in the dispersion.
 8. Fluoropolymer dispersion according to any of the previous claims wherein the fluoropolymer particles comprise polytetrafluoroethylene and/or a melt processible fluoropolymer.
 9. Method of providing a fluoropolymer particle dispersion comprising the steps of: providing a fluoropolymer dispersion comprising fluoropolymer particles having an average particle size of 10 to 400 nm comprising fluorinated surfactant having a molecular weight of less than 1000 g/mol or being free thereof; reducing the amount of said fluorinated surfactant in said dispersion if the amount thereof is more than 0.025% by weight based on the total weight of solids of the dispersion; upconcentrating the fluoropolymer dispersion in the presence of a non-ionic surfactant so as to increase the amount of fluoropolymer solids in said dispersion; and adding an anionic surfactant selected from fluorinated anionic surfactants having a molecular weight of at least 1000 g/mol, non-fluorinated anionic surfactants and mixtures thereof, to the fluoropolymer dispersion prior to or after upconcentrating said fluoropolymer dispersion.
 10. Method according to claim 9 wherein said upconcentration is carried out using ultrafiltration or thermal upconcentration.
 11. Method according to any of the proceeding claims wherein the amount of fluorinated surfactant is reduced to below 0.025% by weight by contacting the fluoropolymer dispersion with an anion exchange resin in the presence of a non-ionic surfactant.
 12. Method according to claim 10 wherein the steps of upconcentration and reduction of the level of said fluorinated surfactant proceed simultaneously.
 13. Method of coating a substrate comprising the step of coating a fluoropolymer dispersion as claimed in any of claims 1 to 8 on said substrate.
 14. Method according to claim 10 wherein said substrate is selected from a metal substrates, glass fiber fabrics, polymeric substrates and paper. 